This invention relates generally to color cathode ray tubes (CRTs) and is particularly directed to an antistatic/antireflective coating for a color CRT having a display screen with a flat outer surface and a curved inner surface.
The glass faceplate of a CRT is comprised of a dielectric material, which operates as a capacitor in storing-up an electrostatic charge as a result of the high voltages applied to the CRT. This electrostatic charge is dissipated to ground for safety reasons. The CRT""s display screen, or faceplate, is typically provided with an antistatic coating on a surface thereof for bleeding the charge to ground. The antistatic coating also contributes to safe operation of the CRT by serving as a shield for electromagnetic radiation generated within the CRT. The antistatic coating may take on various forms such as lithium silicates containing conductive ions, semi-conductor materials such as of tin oxides, or hygroscopic materials which include ions which tend to absorb water vapor rendering the material conductive.
The antistatic coating, or layer, may also be provided with an antireflection capability or it may be used with a separate antireflective layer on the glass display screen of the CRT. Reflected light on the display screen makes it more difficult to view a video image produced by the CRT. Various approaches have been developed to reduce the loss of video image contrast due to CRT display screen glare, which is caused by random scattering of reflected light. A common antireflective coating employs negative reflected light interference wherein reflected light coming from the coating surface and the glass surface under the coating cancel each other for minimizing light reflection. A common current approach makes use of an inner layer comprised of an antimony-doped tin oxide (ATO) or indium-doped tin oxide (ITO) solution. The outer antireflective layer is typically comprised of silica.
Most prior CRT display screens are curvilinear in shape and have a generally constant thickness. Light transmission of a video image through the faceplate is generally constant from the faceplate""s center to its outer periphery provided that the antistatic/antireflective coating applied to the CRT""s display screen is generally uniform in thickness. However, CRTs having a display screen with a flat outer surface are gaining increasing acceptance. In order to ensure accurate positioning of electron beam landing on the CRT""s display screen, the inner surface of the display screen is provided with a curvilinear, i.e., generally concave, shape. The thickness of a flat display screen is thus not uniform from its center to its peripheral edges. With the thickness of the display screen increasing from its center to its peripheral edges, light transmission through the display screen varies substantially over the entire surface of the display screen. In order to minimize this light transmission variation through the display screen, the display screen is typically formed of glass having a very high light transmission characteristic. A CRT display screen having a high light transmission characteristic limits the contrast of and reduces the overall quality of the video image presented on the display screen.
The present invention addresses the aforementioned limitations of the prior art by providing an antistatic/antireflective coating particularly adapted for use on the flat glass display screen of a CRT which compensates for variations in light transmission through the display screen to provide a high degree of video image contrast on the display screen.
Accordingly, it is an object of the present invention to provide an antistatic/antireflective coating for the display screen of a cathode ray tube (CRT) with selectably adjustable light transmission for improved video image presentation on the display screen.
It is another object of the present invention to provide an antistatic/antireflective coating particularly adapted for use on CRT glass display screens having a flat outer surface and a concave inner surface which corrects for light transmission variation through the display screen for improved video image presentation.
A further object of the present invention is to provide an optical coating for the surface of a video display screen having a variable thickness which compensates for differences in light transmission through the display screen to provide uniform light transmission for improved video image contrast.
This invention contemplates a coating for use on a flat outer surface of a glass display screen of a self-emitting display device, wherein the glass display screen further includes a concave inner surface with a phosphor coating thereon, wherein light transmission through the display screen varies with the thickness of the display screen and the display screen has a high light transmission coefficient, and wherein the phosphor coating is responsive to energetic electrons incident thereon for providing a video image, the coating comprising: a first inner electrically conductive antistatic layer disposed on the flat outer surface of the glass display screen and including carbon black particles, wherein light transmission through the first inner antistatic layer is adjustable by varying the concentration of the carbon black particles for providing substantially uniform light transmission of the video image through the display screen and first antistatic layer over the entire surface of the display screen; and a second outer antireflective layer disposed on the first inner antistatic layer and having a low light refractive index.